1. Field of the Invention
The present invention relates to a radiation-sensitive resist composition which is excellent in sensitivity, heat resistance and film thickness retention.
2. Description of the Related Art
A radiation-sensitive resist composition containing a compound having a quinone diazide group finds use as a positive resist, because upon exposure to light having a wavelength of not longer than 500 nm, the quinone diazide group decomposes to form a carboxyl group whereby the originally alkali-insoluble composition becomes alkali-soluble. The positive resist composition has much better resolution than a negative resist composition and is used in the production of integrated circuits such as IC or LSI.
Recently, particularly in the production of integrated circuits, miniaturization has proceeded as the integration level has increased, which results in demands for formation of patterns of submicron order. According to conventional processes for the production of integrated circuits, light exposure is accomplished by placing a mask in intimate contact to a substrate, e.g. a silicon wafer. It is said that this process cannot make patterns thinner than 2 .mu.m. Instead of such conventional processes, the reduction projection in exposure system attracts attention. According to this new system, a pattern of a master mask (reticle) is projected on the substrate with reduction by a lens system, whereby exposure is accomplished. This system realizes a resolving power of submicron.
One of the serious problems in this system is low throughput. Namely, in this system, the total exposure time to expose a wafer is very long because of divided and repeated light exposure unlike a batch light exposure system which is employed in the conventional mask contact printing methods.
To solve this problem, not only an improvement in the apparatus but also an increase in sensitivity of the resist to be used are important.
To increase the sensitivity, it is easiest to decrease molecular weights of a quinone diazide compound and an alkali-soluble resin, in particular, a novolak resin used in the positive photoresist. The decrease of the alkali-soluble resin molecular weight accelerates dissolution of the photoresist in an alkaline developer so that the apparent sensitivity of the photoresist is increased.
This method, however, has very serious disadvantages such as large film thickness loss in an unexposed area (reduction of so-called film thickness retention), worsening a shape of the pattern, and decrease of the .gamma.-value, namely decrease of resolution because of small difference in the dissolving rates in the developing solution between the exposed area and the unexposed area. In addition, when the alkali-soluble resin has a low molecular weight, the resist composition has low heat resistance.
Other measures to increase the sensitivity of the resist are prolongation of a developing time and increase of an alkali concentration in a developing solution. By these measures, though the apparent sensitivity of the photoresist is increased because of increase of solubility of the resist in the developing solution, the film thickness retention is deteriorated, which results in decrease of resolution.
In general, it is impossible to improve the sensitivity while maintaining heat resistance and the film thickness retention, and vice versa.